Many devices, such as protective passive restraints or air bags used in motor vehicles, escape slide chutes, life rafts, and the like, are normally stored in a deflated state and are inflated with gas substantially instantaneously at the time of need. Such devices are generally stored and used in close proximity to humans and, therefore, must be designed with a high safety factor which is effective under all conceivable ambient conditions.
Inflation is sometimes accomplished solely by means of a gas generative composition. At other times, inflation is accomplished by means of a gas, such as air, nitrogen, carbon dioxide, helium, and the like, which is stored under pressure and further pressurized and supplemented at the time of use by the addition of high temperature combustion gas products produced by the burning of a gas-generative composition.
It is, of course, critical that the gas-generative composition be capable of safe and reliable storage without decomposition or ignition at temperatures which are likely to be encountered in a motor vehicle or other storage environment. For example, temperatures as high as about 107.degree. C. (225.degree. C.) may reasonably be experienced. It is also important that substantially all the combustion products generated during use be non-toxic, non-corrosive, non-flammable, particularly where the inflator device is sued in a closed environment, such as a passenger compartment of a motor vehicle.
Igniters are well known in the art for igniting gas-generative compositions in inflators for protective passive restraints used in motor vehicles. In a typical inflator device, the igniter itself may be ignited either directly, or indirectly via an intermediate or auxiliary igniter, by an electrically activated initiator (e.g., squib) which is responds to a sensed impact of the motor vehicle.
Due to the emphasis on weight reduction for improving fuel mileage in motor vehicles, inflator units are often formed form light weight materials, such as aluminum, that can lose strength and mechanical integrity at temperatures significantly above the normal operating temperature of the unit. Although the temperature required for the unit to lose strength and mechanical integrity is much higher than will be encountered in normal vehicle use, these temperatures are readily reached in, for example, a vehicle fire. As the operating pressure of the standard pyrotechnics increases with increasing temperature, a gas generator composition at its autoignition temperature will produce an operating pressure that is too high for a pressure vessel that was designed for minimum weight. Moreover, the melting point of many gas generator compositions is low enough for the gas generator composition to be molten at the autoignition temperature of the composition, which can result in a loss of ballistic control and excessive operating pressures. Therefore, in a vehicle fire, for example, the ignition of the gas generator composition can result in an explosion in which fragments of the inflation unit are propelled at dangerous and potentially lethal velocities.
To prevent such explosions, inflator units have typically been provided with an autoignition propellant (sometimes abbreviated hereinafter as "AIP") that will autoignite and initiate the combustion of the main gas generating pyrotechnic charge at a temperature below that at which the shell or housing of the inflator unit begins to soften and lose structural integrity.
The art is replete with various proposals for AIP compositions such as those described in U.S. Pat. Nos. 4,561,675; 5,084,118; 5,380,380; 5,460,671; 5,739,460 and 5,763,821 (the entire content of each such prior-issued U.S. Patent being incorporated hereinto expressly by reference). Such conventional AIP compositions, however, typically include chlorate compounds, usually in the form of an alkali or alkaline earth metal chlorate (e.g., KClO.sub.3) as an oxidizer. Such chlorate oxidizers are, however, undesirable in some pyrotechnic formulations due to their high degree of sensitivity and incompatibility with other formulation components, especially where the potential for formation of ammonium chlorate exists.
Thus, it would be desirable if an AIP composition could be provided which satisfies the need to reduce the ignition temperature of the propellant composition below its autoignition temperature while, at the same time, exhibits a high degree of stability and compatibility. It is towards fulfilling such needs that the present invention is directed.
Broadly, the present invention is directed to substantially chlorate-free autoignition compositions. More specifically, the present invention is preferably embodied in substantially chlorate-free autoignition compositions comprised of (i) an azodiformamidine dinitrate (ADFD), a novel self-deflagrating, low ignition temperature fuel, (ii) an oxidizer (e.g., a perchlorate, nitrate or mixture thereof) and (iii) an ignition accelerator/augmentor (e.g., a metal or metal oxide powder). One especially preferred AIP composition in accordance with the present invention includes ADFD, a mixture of ammonium perchlorate and sodium nitrate and an iron oxide powder.
These, as well as other, aspects and advantages of the present invention will become more clear from the following detailed description of the preferred exemplary embodiments thereof which follows.